Re: LF: OPERA Question

[M0FMT <m0fmt@yahoo.co.uk>]

Hi Graham

I am as bad not keen on WSPR but this Opera mode has court my attention primarily because IMHO it suits the LF type TX gear in common with CW, QRSS and FSK. And it has proved to be very effective. I have used my RX filters down to 200Hz and found by experiment that it was best to leave the IF at 2.4+/-kHz for best results. Didn't know exactly why it should be better but felt must be a function of the DSP software.  So thanks for the conformation.

I think there is a limit to listening through a narrow CW filter  I have used 50Hz but the keying sound is very "soft" and even at moderate speeds in my experience becomes less easy to copy....... speaking as a 5WPMer.

 

73 es GL Pete M0FMT IO91UX

From: Graham <g8fzk@g8fzk.fsnet.co.uk>
To: rsgb_lf_group@blacksheep.org
Sent: Thursday, 12 July 2012, 0:43
Subject: Re: LF: OPERA Question

Each to  his  own Pete,

 

Really  its  only  slowly that  the  amount of  work and  technical  input that's  needed  to  produce  one of these numeric  based  data  modes is  becoming  apparent , odd  as you  say some of the  fire fights  over  mode V mode , cpu loading  etc 

 

A simple  question  over  filter  bandwidth reveals   yet  another  layer of  maths  and  intrigue, based round  the  definition  of  the  word 'filter'  weather  you are  referring to a  analogue  device  or a  computational  routine  that's  deployed  to  provide a  best  fit  of the  recovered  'pulse' ... reminds  me  of  a episode of the  Avengers  , every time  you open a  door  the  room  has changed  , in this  case the  words  have  different  meanings . . . same thing  with the  spread spectrum knees  up ,  spreading  routines  applied  to  data  , nothing  to  do  with  radio  bandwidth ..

 

In simplicity , the  answer from Jose  was  'Do  nothing  to the  signal , leave  all  filtering  to the software'    selecting  a  narrow  filter  adds   phase shift , time delay  and  additional  noise ....... so  direct  conversion  SDR  is the  optimum  Rx  system !

 

The  main  thing  is  'we'  have  some  pretty robust  data  modes to  play  with  on here  , activity is  increasing  by the  day  and  its  only   July  !  

 

73 - G ..

 

From: M0FMT

Sent: Wednesday, July 11, 2012 11:09 PM

To: rsgb_lf_group@blacksheep.org

Subject: Re: LF: OPERA Question

Hi Graham

 

SDR; doing your best to drag them kicking and screaming into the future. I don't get the resistance..... the QEX (ARRL) files are available to all (unlike other national outfits) on DSP, a technology which has been with us for a decade or two! But easier to implement these days.

73 es GL Pete M0FMT IO91UX

From: Graham <g8fzk@g8fzk.fsnet.co.uk>
To: rsgb_lf_group@blacksheep.org
Cc: james.moritz@btopenworld.com
Sent: Wednesday, 11 July 2012, 16:51
Subject: Re: LF: OPERA Question

''At first glance it appears crazy''

Mike,

Welcome to the crazy world of  JR  !

The  basic  explanation is the  narrow filters  increase the  noise power in a limited  bandwidth , ringing  due to the  Q , similar to  CW , narrow filters tend to  round  the  CW signal

The DSP  engine  is  better equipped to  differentiate between  carrier and noise and  has a  much  greater  dynamic  range , so  optimum results  are obtained , when the  signal is  presented  to the  interface, as close to to the  original as  possible , the  DSP filter profiles are  tailored to the mode/speed in use

This can be  noticed  with the  new  generation of  SDR  support software , where  audio/voice recovery  can be  superior to conventional hardware based systems

With hardware filtering , there  are  transit  (group) delays which  can alter the  amplitude / time  , either  from  on/off  keying  or  with FSK, this can  be  observed when  sending  wide  band  FSK , although  the  audio level  remains  constant and  'phase continuous'  , its possible  some times to  see a  'am modulation'  envelope  on the  carrier    ... $$$$  can solve this  problem , but  for  most  Ham  kit its  something  that 'happens'

Physical  constraints , if  there  is  a  very large  carrier in the  pass band  and  this  is pushing  the  hardware into  non-linearity / A/D  to over  range  , giving  quantising  errors, then  , yes  filtering would help , but  its more  likely  its the  analogue  path  that's  causing intermod products .....reducing  the  rf/if gain will  provide the  solution

I think that's  about the  picture , if  Jim's  about , im sure  he  will fill in the  gaps

73 -G..




--------------------------------------------------
From: "Mike Dennison" <mike.dennison@ntlworld.com>
Sent: Wednesday, July 11, 2012 10:18 AM
To: <rsgb_lf_group@blacksheep.org>; <rsgb_lf_group@yahoogroups.co.uk>
Subject: Re: LF:  OPERA  Question

>> Narrow  IF filters  are  not  desirable  and  reduce the
>> performance of the  demodulator  , better  simply  use  SSB  filter
>> GL ..73 -G..
>
> Graham,
>
> Why is that the case? What does the considerable extra bandwidth
> achieve? Is the SSB bandwidth optimal, or would it be even better
> with 10kHz bandwidth, or 100kHz?. Why is 3kHz better than perhaps
> 1kHz or 2kHz?
>
> At first glance it appears crazy to let in all sorts of adjacent
> channel QRM (the bandwidth is more than ten times the size of the
> entire Opera window) when using a mode that occupies a fraction of
> 1Hz.
>
> Is it simply that the 1.7kHz Tx tone is high enough for the SSB
> filter to kill its harmonics, and on receive it is difficult to get a
> 1.7kHz tone out of a CW filter, even with passband shifting.
>
> Am I missing something?
>
> 73 de Mike, G3XDV
> g3xdv.blogspot.co.uk
> ================
>
>
>